U.S. patent application number 12/341166 was filed with the patent office on 2010-06-24 for arc detection using detailed and approximate coefficients from discrete wavelet transforms.
This patent application is currently assigned to General Electric Company. Invention is credited to Sriram Changali, Konstantin Vladimir Grigoryan, Scott Jeffrey Hall, John Kenneth Hooker.
Application Number | 20100157488 12/341166 |
Document ID | / |
Family ID | 42044434 |
Filed Date | 2010-06-24 |
United States Patent
Application |
20100157488 |
Kind Code |
A1 |
Hall; Scott Jeffrey ; et
al. |
June 24, 2010 |
ARC DETECTION USING DETAILED AND APPROXIMATE COEFFICIENTS FROM
DISCRETE WAVELET TRANSFORMS
Abstract
An apparatus for facilitating interruption of current in an
electrical circuit is provided and includes a current sensing
device in the electrical circuit to service an electrical load, the
current sensing device being productive of an output signal
representative of a load current passing therethrough, a detection
unit, in signal communication with the current sensing device such
that the output signal is received by the detection unit, the
detection unit being configured and disposed to output a secondary
signal based on the output signal, and a microcontroller to receive
and to decompose the secondary signal into detailed and approximate
coefficients, and to generate a trip signal for use in interrupting
an operation of the electrical circuit when a current of the sensed
load is above a predetermined threshold and the detailed and
approximate coefficients cooperatively indicate that threshold
conditions for trip signal generation are satisfied.
Inventors: |
Hall; Scott Jeffrey;
(Louisville, KY) ; Hooker; John Kenneth;
(Louisville, KY) ; Grigoryan; Konstantin Vladimir;
(Louisville, KY) ; Changali; Sriram; (Cochin,
IN) |
Correspondence
Address: |
CANTOR COLBURN, LLP
20 Church Street, 22nd Floor
Hartford
CT
06103
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
42044434 |
Appl. No.: |
12/341166 |
Filed: |
December 22, 2008 |
Current U.S.
Class: |
361/42 ;
361/93.1 |
Current CPC
Class: |
H02H 1/0092 20130101;
H02H 1/0015 20130101 |
Class at
Publication: |
361/42 ;
361/93.1 |
International
Class: |
H02H 3/08 20060101
H02H003/08 |
Claims
1. An apparatus for facilitating interruption of current in an
electrical circuit, the apparatus comprising: a current sensing
device disposed in the electrical circuit to service an electrical
load, the current sensing device being productive of an output
signal representative of a load current passing therethrough; a
detection unit, in signal communication with the current sensing
device such that the output signal produced by the current sensing
device is received by the detection unit, the detection unit being
configured and disposed to output a secondary signal based on the
output signal; and a microcontroller, coupled to the detection
unit, being responsive to computer executable instructions which,
when executed by the microcontroller, cause the microcontroller to
receive and to decompose the secondary signal into detailed and
approximate coefficients, and to generate a trip signal for use in
interrupting an operation of the electrical circuit when a current
of the sensed load is above a predetermined threshold and the
detailed and approximate coefficients cooperatively indicate that
threshold conditions for trip signal generation are satisfied.
2. The apparatus according to claim 1, further comprising a current
measurement unit, configured to output to the microcontroller a
measurement signal, which is related to a timing of the output
signal and which is employed by the microcontroller in a
determination of whether the threshold conditions are indicated as
being satisfied.
3. The apparatus according to claim 1, further comprising an
ambient temperature sensor coupled to the microcontroller by which
the microcontroller determines whether to compensate for
temperature changes of the current sensing device.
4. The apparatus according to claim 1, further comprising a summing
amplifier, including a filter, operably disposed between the
current sensing device and the detection unit and in signal
communication with the microcontroller, wherein the filter of the
summing amplifier and the detection unit cooperatively filter
components of the secondary signal having frequencies outside of a
range of about 1-250 kHz from the secondary signal.
5. The apparatus according to claim 1, wherein the detailed
coefficients are obtained from components of the secondary signal
having frequencies in a range of about 75-250 kHz, and the
approximate coefficients are obtained from components of the
secondary signal having frequencies in a range of about 1-75
kHz.
6. The apparatus according to claim 1, wherein the microcontroller
is further responsive to computer executable instructions which,
when executed by the microcontroller, cause the microcontroller to
introduce a deadband into the signal before the decomposition of
the secondary signal.
7. The apparatus according to claim 1, further comprising a
detection circuit, disposed in signal communication with the
microcontroller, wherein the detection circuit is further
configured and disposed to detect a zero cross instance of the
output signal and to instruct the microcontroller to subsequently
decompose the secondary signal.
8. The apparatus according to claim 1, wherein, if the current of
the sensed load exceeds a predetermined amperage, the
microcontroller generates the trip signal when the detailed
coefficients indicate that threshold conditions for trip signal
generation are satisfied at a predetermined frequency for a given
period of time.
9. A computer implemented method of performing arc fault current
interruption (AFCI) for a circuit, the method comprising: sensing a
load current at a current sensing device in electrical
communication with the circuit; generating a secondary signal
reflective of a current of the sensed load current at a detection
unit in signal communication with the current sensing device;
sampling the secondary signal at a first predetermined frequency at
a microcontroller coupled to the detection unit; when the sampling
of the secondary signal is determined to be complete and when a
zero cross of the secondary signal is determined to have been
sampled, computing detailed coefficients from first components of
the secondary signal and approximate coefficients from second
components of the secondary signal; determining if threshold
criteria are determined to have been met based on the first
coefficients or, if the sensed load current is below a
predetermined threshold, based on the detailed and approximate
coefficients; and, if so, issuing a trip signal to interrupt an
operation of the circuit.
10. The method according to claim 9, wherein the detailed
components of the secondary signal have characteristic frequencies
which are higher than those of the approximate components of the
secondary signal.
11. The method according to claim 9, further comprising: sampling
the secondary signal at a second predetermined frequency, which is
lower than the first predetermined frequency, while the sampling of
the secondary signal at the first predetermined frequency is not
complete; and computing a rolling average of the second
predetermined frequency sampled secondary signal when the second
predetermined frequency sampling is complete.
12. The method according to claim 11, further comprising computing
the detailed and approximate coefficients independent of the
rolling average.
13. The method according to claim 11, wherein, if the sampling of
the secondary signal at the second predetermined frequency is
determined to not be complete or if the predetermined threshold is
determined to have not been met, continuing the sampling of the
secondary signal at the first predetermined frequency.
14. The method according to claim 11, wherein the sampling of the
secondary signal at the first predetermined frequency occurs in
accordance with a conversion of the sensed load current into the
secondary signal and a determination of whether a root mean square
(RMS) length of the secondary signal is sampled.
15. The method according to claim 14, further comprising
determining whether the secondary signal registers either a
positive or negative zero cross subsequent to the determination of
whether the RMS length is sampled.
16. The method according to claim 15, further comprising: setting
delays for positive or negative zero cross for zero cross secondary
signal sampling; and triggering the sampling of the secondary
signal at the first predetermined frequency when the secondary
signal registers a positive or negative zero cross.
17. A computer implemented method of performing arc fault current
interruption (AFCI) for a circuit, the method comprising:
decomposing first and second portions of a secondary signal, which
is generated at a detection unit as being based on a load current
sensed by a current sensing device with which the detection unit is
in signal communication, into detailed and approximate
coefficients, respectively, using discrete wavelet transforms; with
the first portion of the secondary signal determined to have been
zero cross sampled, computing a sum of absolute values of the
detailed coefficients and computing absolute values and a ratio of
sums thereof of the approximate coefficients for first and second
windows of the secondary signal; comparing the sum of the absolute
values with a first predetermined threshold or, if a current of the
sensed load is below a pre-selected magnitude, comparing a product
of the sum of the absolute values and the ratio of sums with a
second predetermined threshold; and issuing a trip signal to
interrupt an operation of the circuit if a result of the comparison
indicates that the corresponding one of the first and second
predetermined thresholds is exceeded with a predetermined frequency
over a given period of time.
Description
BACKGROUND OF THE INVENTION
[0001] Aspects of the present invention are directed to electrical
systems and, more particularly, to methods and systems for arc
detection in electrical systems.
BRIEF DESCRIPTION OF THE BACKGROUND
[0002] Electrical systems in residential, commercial, and
industrial applications usually include a panel board for receiving
electrical power from a utility source. The received power is then
routed through the panel board to one or more current interrupters
such as, but not limited to circuit breakers, trip units, and
others.
[0003] Each current interrupter distributes the power to a
designated branch, where each branch supplies one or more loads
with the power. The current interrupters are configured to
interrupt the power to the particular branch if certain power
conditions in that branch reach a predetermined set point.
[0004] For example, some current interrupters can interrupt power
due to a ground fault and are commonly known as ground fault
current interrupters (GFCIs). The ground fault condition results
when an imbalance of current flows between a line conductor and a
neutral conductor and could be caused by a leakage of current or an
arcing fault to ground.
[0005] Other current interrupters can interrupt power due to an
arcing fault and are commonly known as arc fault current
interrupters (AFCIs). Arcing faults may be generally defined as
either series arcs or parallel arcs. Series arcs can occur, for
example, when current passes across a gap in a single conductor.
Parallel arcs, on the other hand, can occur when current passes
between two conductors. Unfortunately, both types of arcing faults
may, for various reasons, not cause a conventional current
interrupter to trip. This is particularly true when a series arc
occurs because the current sensing device in the current
interrupter is unable to distinguish between a series arc and a
normal load current.
SUMMARY OF THE INVENTION
[0006] In accordance with an aspect of the invention, an apparatus
for facilitating interruption of current in an electrical circuit
is provided and includes a current sensing device disposed in the
electrical circuit to service an electrical load, the current
sensing device being productive of an output signal representative
of a load current passing therethrough, a detection unit, in signal
communication with the current sensing device such that the output
signal produced by the current sensing device is received by the
detection unit, the detection unit being configured and disposed to
output a secondary signal based on the output signal, and a
microcontroller, coupled to the detection unit, being responsive to
computer executable instructions which, when executed by the
microcontroller, cause the microcontroller to receive and to
decompose the secondary signal into detailed and approximate
coefficients, and to generate a trip signal for use in interrupting
an operation of the electrical circuit when a current of the sensed
load is above a predetermined threshold and the detailed and
approximate coefficients cooperatively indicate that threshold
conditions for trip signal generation are satisfied.
[0007] In accordance with another aspect of the invention, a
computer implemented method of performing arc fault current
interruption (AFCI) for a circuit is provided and includes sensing
a load current at a current sensing device in electrical
communication with the circuit, generating a secondary signal
reflective of a current of the sensed load current at a detection
unit in signal communication with the current sensing device,
sampling the secondary signal at a first predetermined frequency at
a microcontroller coupled to the detection unit, when the sampling
of the secondary signal is determined to be complete and when a
zero cross of the secondary signal is determined to have been
sampled, computing detailed coefficients from first components of
the secondary signal and approximate coefficients from second
components of the secondary signal, determining if threshold
criteria are determined to have been met based on the first
coefficients or, if the sensed load current is below a
predetermined threshold, based on the detailed and approximate
coefficients, and, if so, issuing a trip signal to interrupt an
operation of the circuit.
[0008] In accordance with another aspect of the invention, a
computer implemented method of performing arc fault current
interruption (AFCI) for a circuit is provided and includes
decomposing first and second portions of a secondary signal, which
is generated at a detection unit as being based on a load current
sensed by a current sensing device with which the detection unit is
in signal communication, into detailed and approximate
coefficients, respectively, using discrete wavelet transforms, with
the first portion of the secondary signal determined to have been
zero cross sampled, computing a sum of absolute values of the
detailed coefficients and computing absolute values and a ratio of
sums thereof of the approximate coefficients for first and second
windows of the secondary signal, comparing the sum of the absolute
values with a first predetermined threshold or, if a current of the
sensed load is below a pre-selected magnitude, comparing a product
of the sum of the absolute values and the ratio of sums with a
second predetermined threshold, and issuing a trip signal to
interrupt an operation of the circuit if a result of the comparison
indicates that the corresponding one of the first and second
predetermined thresholds is exceeded with a predetermined frequency
over a given period of time.
[0009] Additional features and advantages are realized through the
techniques of the present invention. Other embodiments and aspects
of the invention are described in detail herein and are considered
a part of the claimed invention. For a better understanding of the
invention with advantages and features, refer to the description
and to the drawings.
BRIEF DESCRIPTIONS OF THE DRAWINGS
[0010] The subject matter regarded as the invention is particularly
pointed out and distinctly claimed in the claims at the conclusion
of the specification. The foregoing and other aspects, features,
and advantages of the invention are apparent from the following
detailed description taken in conjunction with the accompanying
drawings in which:
[0011] FIG. 1 is a schematic diagram of a microcontroller based
combination arc fault current interrupter;
[0012] FIG. 2 is a graph of a signal based upon a current of a
sensed load in a circuit to which the arc fault current interrupter
of FIG. 1 is coupled;
[0013] FIG. 3 is a flow diagram of a trip signal issuing
algorithm;
[0014] FIG. 4 is a flow diagram illustrating an interrupt handling
algorithm;
[0015] FIG. 5 is a flow diagram illustrating discrete wavelet
coefficient computing algorithms according to an embodiment of the
invention; and
[0016] FIG. 6 is a flow diagram illustrating discrete wavelet
coefficient computing algorithms according to another embodiment of
the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] With reference to FIG. 1, an apparatus for facilitating
interruption of current in an electrical circuit by performing arc
fault current interruption (AFCI) is provided and includes a
current sensing device 10, such as a bimetal, hall effect sensors,
MEMs, CTs etc, which is configured to sense a load from which a
current signal is generated. The current sensing device 10 may be
formed of resistive materials that have a characteristic resistance
at room temperature of 6 mOhms (milli-ohms) (at 15 A) or 3 mOhms
(at 20 A). The current sensing device 10 is electrically coupled to
a signal line along which a summing amplifier 20, including a low
pass filter, is disposed. The current signal, therefore, flows from
the current sensing device 10 and to the summing amplifier 20 along
with a test signal 90 that may be outputted by a microcontroller
80.
[0018] While embodiments of the invention are disclosed having a
bimetal as an example current sensing device 10, it will be
appreciated that the scope of the invention is not so limited and
also encompasses other resistive elements suitable for the purposes
disclosed herein, such as, for example, brass, bronze, copper
alloy, steel, stainless steel, inconel steel and/or carbon-steel
alloys.
[0019] The signal line is coupled to a first arc detection unit 30,
such as a series arc detection unit 30, a current measurement unit
50, such as a root mean square current measurement unit, a p-p
current measurement unit, a Hall effect current sensor or any other
suitable device, and, optionally, a second arc detection unit 40,
such as a parallel arc detection unit 40. The first arc detection
unit 30 is configured to output a secondary signal to the
microcontroller 80 for use in detecting an arcing condition (e.g.,
a series arc) in the sensed load current. The second arc detection
unit 40 is similarly configured to output an additional secondary
signal to the microcontroller 80 for use in detecting an arcing
condition (e.g., a parallel arc) in the sensed load current. The
current measurement unit 50 is configured to detect a current in
the sensed load current and to output a yet another secondary
signal to the microcontroller 80, which is based on the detected
current, for use in the performance of, e.g., current dependent
offset calculations, RMS current measurement and arc detection
sample timing.
[0020] In the present context, series and parallel arcs refer to
electric breakdowns of a normally nonconductive media, such as air,
that produce luminous electrical discharges, such as sparks, which
result from current flowing through the normally nonconductive
media. Series arcs occur in series with the load current where, as
an example, a current carrying line is broken. As such, series arc
current can be no higher than the load current. Conversely,
parallel arcs occur between oppositely charged conductors, such as
a circuit and a grounded element, and may be characterized by high
current spikes and little or no load impedance.
[0021] The first arc detection unit 30 operates at a sampling rate
of 300 kHz and filters all but those signals having frequencies of
about 1 kHz-250 kHz from the current signal. To this end, the first
arc detection unit 30 includes a high pass filter 31 which operates
cooperatively with the low pass filter of the summing amplifier 20.
Where, the second arc detection unit 40 is employed, the second arc
detection unit 40 operates at a sampling rate of 10 kHz and filters
all but those sub-signals having frequencies of about 150-900 Hz
from the current signal. To this end, the second arc detection unit
40 includes a low pass filter 41 and a high pass filter 42. The
current measurement unit 50 operates at a sampling rate of 10 kHz
and includes a low pass filter 51.
[0022] The microcontroller 80 is configured to decompose at least
the secondary signal received from the first arc detection unit 30.
The decomposition is accomplished via discrete wavelet transforms
DWTs, such as mother wavelets, which are obtained from external
computations and at least partly from information contained within
the signal received from the current measurement unit 50.
[0023] A result of the decomposition is the further filtering of
the secondary signal received from the first arc detection unit 30
into first and second frequency components in which, in an
embodiment of the invention, the first frequency is higher than the
second frequency. That is, the secondary signal is decomposed into
a first or high frequency component including those portions
thereof having frequencies of about 75-250 kHz, from which first
coefficients (hereinafter referred to as "detailed coefficients")
are obtained, and a second or low frequency component including
those portions thereof having frequencies of about .about.1-75 kHz,
from which first and second sets of second coefficients
(hereinafter referred to as "approximate coefficients") are
obtained.
[0024] Here, with reference to FIG. 2, it is noted that the
decomposition of the secondary signal into the low frequency
component signal results in a more pronounced "shoulder" section
thereof As such, the high to low signal content characterized by an
arcing condition is relatively easily captured. This is depicted in
the first window, which is reflective of the "arc extinguish"
condition of the secondary signal, and the second window, which is
reflective of the "shoulder" section of the secondary signal. Here,
the sizes and positions of the first and second windows are arrived
at by optimization calculations. It is further noted that, since a
signal content tends to shift as current increases, the usefulness
of the second frequency component of the secondary signal decreases
as a current of the sensed load increases.
[0025] Once the detailed and approximate coefficients are obtained,
as discussed below with reference to FIGS. 4 and 5, the
microcontroller 80 calculates a sum of absolute values of the
detailed coefficients (SumCD), a current dependent offset of the
sum (SumCD_offset), which depends upon the current of the sensed
load as detected by the current measurement unit 50, and a ratio of
first and second sums of absolute values of the approximate
coefficients (Ratio). The first sum of the absolute values of the
approximate coefficients are calculated from the first set of
approximate coefficients, which are obtained from the first window
of FIG. 2, and the second sum of the absolute values of the
approximate coefficients are calculated from the second set of
approximate coefficients, which are obtained from the second window
of FIG. 2.
[0026] The microcontroller 80 generates a trip signal S.sub.T, as
shown in FIG. 1, when a product (Product) of the SumCD minus the
SumCD_offset and the Ratio indicates, a predetermined number of
times during a given time period, that one or more threshold
conditions for trip signal S.sub.T generation are satisfied. Here,
an exemplary threshold condition may refer to a signal measurement,
which indicates that an arcing condition in the sensed load
occurs.
[0027] In detail, where the current of the sensed load is below 15
Amps and, if the Product has a value that is greater than 300, the
one or more threshold conditions for trip signal S.sub.T generation
are indicated as being satisfied. Here, SumCD is calculated from
the high frequency component signal, SumCD_offset is calculated
from the RMS current multiplied by 20 and Ratio is calculated from
the low frequency component signal. If SumCD is less than SumCD
minus SumCD_offset, the Product is calculated as having a value of
zero and it is determined that the one or more threshold conditions
for trip signal S.sub.T generation are not satisfied. If, however,
SumCD is greater than SumCD minus SumCD_offset, the Product is
calculated as being equal to the product of SumCD minus
SumCD_offset and the Ratio and it is determined that the one or
more threshold conditions for trip signal S.sub.T generation are
satisfied if the Product has a value that is greater than 300.
[0028] Where the current is greater than 15 Amps, since a signal
content tends to shift as current increases, the usefulness of the
low frequency component decreases, as mentioned above, only the
value of SumCD is used to determine whether the one or more
threshold conditions for trip signal S.sub.T generation are
satisfied. That is, for currents between 15 and 22.5 Amps, the one
or more threshold conditions for trip signal S.sub.T generation are
satisfied if SumCD has a value that is greater than 300. Similarly,
for currents above 22.5 Amps, the one or more threshold conditions
for trip signal S.sub.T generation are satisfied if SumCD has a
value that is greater than 400.
[0029] In accordance with current regulations, required trip times
are given as in the following Table 1:
TABLE-US-00001 TABLE 1 Test current, Amperes.sup.c 15 Amp AFCI 20
Amp AFCI 30 Amp AFCI 5 1 sec 1 sec 1 sec 10 0.4 sec 0.4 sec 0.4 sec
Rated current 0.28 sec 0.2 sec 0.14 sec 150% rated current 0.16
sec.sup.a 0.11 sec.sup.a .1 sec 0.19 sec.sup.b 0.14 sec.sup.b
.sup.aRequired clearing time when switch is closed on the load side
of the AFCI .sup.bRequired clearing time when the AFCI is closed on
the fault .sup.cTests at 120 V are also applicable to cord AFCIs
rated 120 V/240 V
[0030] In order to meet these trip times, it is required that at
least 40% of the cycles in the allotted trip time meet the
conditions discussed above. For example, at 5 Amps, 60 line cycles
occur in 1 second and 60 times 0.4 equals 24 line cycles. Thus,
when 24 or more line cycles out of 60 meet the trip conditions, the
microcontroller 80 will generate the trip signal S.sub.T.
[0031] While embodiments of the invention are disclosed in which a
magnitude of the given current load at which the first or the first
and second coefficients are employed is 15 Amps, it is understand
that other magnitudes may be used as the given current load.
[0032] In accordance with embodiments of the invention, each DWT is
a short wave of finite length that integrates to zero over its
period of existence. The discrete wavelet detailed and approximate
coefficients are obtained from each DWT as follows:
##STR00001##
[0033] where x[n]=an input signal, g[n]=a high pass digital filter
from a mother wavelet, and h[n]=a low pass digital filter from the
mother wavelet.
[0034] Use of the DWTs to obtain the discrete wavelet detailed and
approximate coefficients provides several advantages in current
signal analysis as compared to other analytical tools, such as
Fourier transforms (FT) and Fast Fourier Transforms (FFT). For
example, DWTs provide a measure of a correlation between the mother
wavelet and the current signal. In addition, DWTs can inform as to
what time a particular frequency occurred, are simpler to calculate
and allow for a detection of an extinguish/re-strike event, which
is characteristic to parallel and series arcs, by also allowing for
a search for particular frequencies/patterns at zero cross
moments.
[0035] Thus, when the microcontroller 80 applies DWTs to an arc
detection operation, the microcontroller 80 may operate by
identifying a pattern or a signature that can be associated with
the arcing, selecting a predetermined mother wavelet that
relatively closely correlates with that pattern or signature,
selecting a frequency range to analyze the arcing that provides an
optimized signal-to-noise ratio, selecting a portion of the
waveform as the focus area and selecting the required window(s)
size(s) that corresponds to the selected portion of the
waveform.
[0036] With this in mind, it has been seen that the "Daubechies10"
or "db10" mother wavelet is highly suitable for arc detection where
the frequency range is set at 93 kHz or more, the sampling
frequency is set at 300 kHz and no anti-aliasing filter is applied.
Since it has also been seen that indicators of arcing lie at the
zero cross points of the current signal, the zero cross points
determine when sampling is triggered. Thus, a window size for the
sampling frequency of 300 kHz is set as 25.3 degrees such that at
least one of either the re-strike or extinguish events of an arc
will be caught within the window.
[0037] Still referring to FIG. 1, the apparatus may further include
an ambient temperature sensor 60 that is coupled to the
microcontroller 80. The ambient temperature sensor 60 measures the
ambient temperature of, at least, the current sensing device 10 and
outputs the measurement to the microcontroller 80. The
microcontroller 80 then determines whether to compensate for any
temperature changes of the current sensing device 10 in the
calculations mentioned above.
[0038] In addition, the apparatus may further include a push to
test switch 70 including a series arc test configuration 71 and a
parallel arc test configuration 72. The push to test switch 70 is
coupled to the microcontroller 80 and allows an operator to test
the apparatus upon installation in accordance with local and
non-local regulations.
[0039] The microcontroller 80 may be further configured to
introduce a deadband into the signal before the decomposition
thereof Here, any sampled secondary signal that is in the deadband
is zeroed and, once the signal is outside of the deadband, the
deadband values are subtracted or added depending on whether the
secondary signal has a negative or a positive value. The deadband
is, therefore, configured to reduce a sensitivity of the
microcontroller 80 to analog to digital (A/D) bit dithering.
[0040] The apparatus further includes a detection circuit 100 which
is configured to detect a zero cross instance of the secondary
signal and to instruct the microcontroller 80 to subsequently
decompose the secondary signal as described above. In this
capacity, the detection circuit 100 is coupled to a neutral
electrical source at one side thereof and, at the other side, to an
input of the microcontroller 80.
[0041] With reference now to FIGS. 3-6, a method of performing arc
fault current interruption (AFCI) will be described. As shown in
FIG. 3, upon initialization of the algorithm (operation 100), which
then runs continuously, during which sensing of a load current
occurs at a current sensing device 10, a determination is made as
to whether high frequency sampling of a secondary signal, which is
generated as being based on the sensed load current by a detection
unit in signal communication with the current sensing device 10, is
complete or not (operation 200).
[0042] The high frequency sampling of the secondary signal in
operation 200 occurs in accordance with the interrupt handling
algorithm of FIG. 4. As shown, the interrupt handling algorithm
begins with the receiving at the microcontroller 80 of a low
frequency interrupt signal (operation 201), which is based on the
secondary signal outputted by the current measurement unit 50. At
this point, it is determined whether an RMS length of the secondary
signal has been sampled (operation 202) and, if an RMS length has
been sampled, a value of the RMS is computed (operation 203). Once
the value of the RMS is computed, the value is used to determine
how fast the apparatus needs to trip in the presence of an arcing
condition. If the RMS length has not yet been sampled, input from
the detection circuit 100 is received (operation 204) from which it
is determined whether a positive zero cross has occurred (operation
205).
[0043] If the positive zero cross has not occurred, control returns
to operation 204. If, however, the positive zero cross has
occurred, delays for positive zero cross for zero cross sampling
are set (operation 207). At this point, the high frequency sampling
of operation 200 is triggered.
[0044] Referring back to FIG. 3, if the high frequency sampling of
the secondary signal is determined to not be complete, the
secondary signal is sampled at a low frequency (operation 300) and,
if the low frequency sampling is determined to be complete, a
rolling average of the low frequency sampled signal is computed
(operation 500).
[0045] If the high frequency sampling of the secondary signal is
completed and if a zero cross is determined to have been sampled in
accordance with an output of the detection circuit 100, the zero
cross discrete wavelet detailed and approximate coefficients are
computed independent of the rolling average (operations 410 and
411, respectively). Conversely, if the high frequency sampling of
the secondary signal is complete and if the zero cross is
determined to have not been sampled, control returns to operation
200.
[0046] Here, with reference to FIG. 5, the illustrated discrete
wavelet algorithm is employed in operation 410. As shown, the
sampled signal is initially defined as a signal with the
OuterIndex, which refers to an index for the convoluted signal, the
SumCD, which is the absolute value of the sum of detailed
coefficients, and the InnerIndex, which is an index of a filter in
use, each being set to zero.
[0047] First, whether the OuterIndex is less than a length of the
convoluted signal is determined. If the OuterIndex is not less than
a length of the convoluted signal, a value of the SumCD is returned
to zero. Conversely, if the OuterIndex is less than a length of the
convoluted signal, values of the CDs, which are the individual
detailed coefficients, are set to zero and a value of a JumpIndex
is set to a value of the convoluted signal multiplied by two.
[0048] Then, whether the InnerIndex is less than a length of the
filter is determined. If the InnerIndex is less than a length of
the filter, the values of the CDs are set to the values of the CDs
added to a value of the signal. Here, the signal value is a value
of the JumpIndex added to a value of the InnerIndex multiplied by a
value of the filter. This process is repeated until the InnerIndex
is determined to not be less than a length of the filter. At this
point, the values of the CDs are set to the absolute values of the
CDs and the value of the SumCD is set to the absolute value of the
SumCD added to the values of the CDs.
[0049] With reference to FIG. 6, the illustrated discrete wavelet
algorithm according to another embodiment is employed in operation
411. As shown, it is first determined whether a length of the
sampled signal is equal to a predetermined length minus 45. If so,
values for the OuterIndex, which refers to an index for the
convoluted signal, the SumCA, which is the absolute value of the
sum of approximate coefficients, and the FirstWindowSum and the
SecondWindowSum, which are each values of the sums of the first and
second sets of the absolute values of the approximate coefficients,
are set to zero.
[0050] At this point, it is determined whether the value of the
OuterIndex is less than the predetermined signal length minus 45.
If it is not, the value of the Ratio is set as being equal to the
FirstWindowSum divided by the SecondWindowSum. If the value of the
OuterIndex is less than the predetermined signal length minus 45,
the value of the CA is set to zero and a value of a JumpIndex is
set to the value of the OuterIndex multiplied by two.
[0051] Whether a value of the InnerIndex is less than a length of
Approximate Filters is then determined. If it is, the value of the
CA is set to the value of the CA added to a value of a signal
multiplied a value of an ApproximateFilter, where the value of the
signal is multiplied by the sum of the JumpIndex and the
InnerIndex, and where the value of the ApproximateFilter is
multiplied by the value of the InnerIndex. If the value of the
InnerIndex is not less than a length of Approximate Filters, it is
determined whether the value of the OuterIndex is less than a value
of an end of the first window.
[0052] If the value of the OuterIndex is less than the value of an
end of the first window, the value of the FirstWindowSum is set to
the value of the FirstWindowSum as being added to an absolute value
of the CA and it is again determined whether the value of the
OuterIndex is less than the predetermined signal length minus 45.
If the value of the OuterIndex is not less than the value of the
end of the first window, it is determined whether the value of the
OuterIndex is less than a value of an end of the second window.
Here, if the value of the OuterIndex is not less than the value of
the end of the second window, it is again determined whether the
value of the OuterIndex is less than the predetermined signal
length minus 45. If the value of the OuterIndex is less than the
value of the end of the second window, the value of the
SecondWindowSum is set to the value of the SecondWindowSum added to
an absolute value of the CA and it is again determined whether the
value of the OuterIndex is less than the predetermined signal
length minus 45.
[0053] With reference back to FIG. 3, once the detailed and
approximate coefficients are computed, it is determined whether all
threshold criteria have been met (operation 600), as discussed
above. If all threshold criteria have not been met, control returns
to operation 200. If, however, all threshold criteria have been
met, a trip signal S.sub.T is issued (operation 700).
[0054] An embodiment of the invention may be embodied in the form
of computer-implemented processes and apparatuses for practicing
those processes. The present invention may also be embodied in the
form of a computer program product having computer program code
containing instructions embodied in tangible media, such as floppy
diskettes, CD-ROMs, hard drives, USB (universal serial bus) drives,
or any other computer readable storage medium, such as random
access memory (RAM), read only memory (ROM), or erasable
programmable read only memory (EPROM), for example, wherein, when
the computer program code is loaded into and executed by a
computer, the computer becomes an apparatus for practicing the
invention. The present invention may also be embodied in the form
of computer program code, for example, whether stored in a storage
medium, loaded into and/or executed by a computer, or transmitted
over some transmission medium, such as over electrical wiring or
cabling, through fiber optics, or via electromagnetic radiation,
wherein when the computer program code is loaded into and executed
by a computer, the computer becomes an apparatus for practicing the
invention. When implemented on a general-purpose microprocessor,
the computer program code segments configure the microprocessor to
create specific logic circuits. A technical effect of the
executable instructions is to receive and to decompose a secondary
signal into first and second coefficients, and to generate a trip
signal for use in interrupting an operation of an electrical
circuit when a current of a sensed load is below a predetermined
threshold and the first and second coefficients cooperatively
indicate that threshold conditions for trip signal generation are
satisfied or when a current of the sensed load is above the
predetermined threshold and the first coefficients alone indicate
that the threshold conditions are satisfied.
[0055] While the disclosure has been described with reference to
exemplary embodiments, it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the disclosure. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
disclosure without departing from the essential scope thereof.
Therefore, it is intended that the disclosure not be limited to the
particular exemplary embodiment disclosed as the best mode
contemplated for carrying out this disclosure, but that the
disclosure will include all embodiments falling within the scope of
the appended claims.
* * * * *